This invention relates to an optical waveguide which is needed in an optical integrated circuit used in the technical fields of optical communication and optical processing, and a method of manufacturing the same.
An optical waveguide consists of a core portion provided on a substrate material and adapted to guide light, and a clad portion, the refractive index of which is lower than that of the core portion. No matter how the optical waveguide is constructed, not less than two kinds of transparent materials having different refractive indexes are required, and, moreover, these materials must consist of films. The difference between the refractive indexes of these materials is not more than 2-3% at the highest in most cases. A number of kinds of materials including semiconductors, ferroelectrics, glass and organic materials have been reported as the materials which can be used to manufacture an optical waveguide. A combination of such materials having a suitable refractive index difference has been used selectively in accordance with the purpose of the optical waveguide. However, if the construction of an optical waveguide, the differences between the refractive indexes of parts thereof and the manufacturing method are considered carefully, the number of combinations of film materials which can be used in practice is extremely limited. In other words, the designing of an optical waveguide is reetricted largely by the materials, so that it becomes difficult to design an optical waveguide freely.
This difficulty becomes more remarkable especially in the case where a substrate material in use has a high refractive index because there are a variety of materials which have a comparatively low refractive index, and which include various kinds of glass, whereas there are a very few materials which have a high refractive index.
In order that an optical integrated circuit can perform various operations, such as optical modulation and optical switching, it is necessary to utilize physical phenomena, such as the electro-optic effect and the magneto-optic effect. In order to meet these requirements, it is necessary to use special substrate materials, such as electro-optic crystals and magneto-optic crystals. In general, these crystals have a high refractive index as compared with other materials. For example, gallium arsenide which has attracted widespread attention as a material for an optical integrated circuit has a refractive index of as high as about 3.43 when the wavelength is 1 .mu.m. There are no materials except a ternary material of gallium aluminum arsenide, which have such a high refractive index, and which can be combined with gallium arsenide. However, in order to deposit this ternary material on a substrate of gallium arsenide, there is no choice but to employ a complicated method utilizing the epitaxial growth of a film. It is very difficult to form an optical waveguide of an arbitrary construction by using this method.
Due to the above-mentioned facts, there are few examples of optical switches and optical modulators which have heretofore been made public and which have the configuration of a buried waveguide. A buried waveguide is a most excellent waveguide. It shows a small loss, and is not damaged by extended use, among some types of threedimensional waveguides.
The following references are cited to show the state of the art; (i) J. E. Goell; Applied Optics, Vol. 12 (1973), page 737, (ii) T. Izawa et al; Applied Physics Letter, Vol. 38 (1981), page 483, (iii) D. H. Hensler; Applied Optics, Vol. 10 (1971), page 1037.